Chain to cassette ratio....

Yep, the more expensive kit you use the more cost effective the regular chain replacement approach becomes. On lower end kit the cost pretty much evens out.

could do what i am doing, finding out how long a chain can last...

currently somewhere in excess of 11,000miles over the last 18months.

Change cassette every time if you wear out your chain before you change it. If you change the chain before it has gone too far you can get a few chains out of a cassette. You will know if the cassette has gone if a new chain skips on it or if your chains start wearing out quickly. It's best to find a chain that suits you, I find that some higher end KMC and lower end SRAM models seem to wear out in no time when I use them. I find that Campag chains on Campag kit last a really long time.

Also to my ladt post had a cassette on bike for 6 yes 6 years jst changed chain regually and worked great, imo its a moneymaking racket brought about by manufacturers to make more money,

If i had to change the cassette every two chains, it wud cost me a fortune as chain changed after 1200 miles ie 2 months. Not a hope in hell, okif u hav bottomless pockets

lbs wants you to buy more stuff - big surprise!

I stocked up on some quality sram chains from planet x when they were on offer and happily continue to use the same cassette on the second chain.

I've found 2:1 works perfectly well on my 9 speed Tiagra cassettes with SRAM chains provided you change it as soon as it reaches 0.75 (if not before).

Using a ruler: 10 links = 25.4 cm when new. Up to 25.5 cm is OK. A bit past 25.5 cm you should replace the chain, the cassette should be fine. If it is near 25.7 cm you have left it too long, and the most used sprockets will be too badly worn.

Always put your chain under tension to check it.

you should get 2-3 chains out of a cassette if you change at 0.5 but i would ersonally change at 0.75 and just get 2 chains off one cassette.

your other option is use them until it skips and then replace it, depending on your mileage and how often you ride its not normally that more expensive to replace it once its skipping.

6 and half a dozen tbh

If you change at 0.75 you don't need to change the cassette.

Why not have 2 chains per cassette and swap them over at regular mileage intervals then the wear on the cassette will be more in line with that on the chains.

Technically speaking you should change both as you will find the new chain will wear out faster. That said it’s a rather expensive way of doing things and if perfect meshing isn’t high on your list i.e. pro racer I wouldn’t worry about it and carry on the way you are doing it two chains per cassette. No doubt someone else will give you different advice but it works for me.